There exist a number of areas in the world where interconnections between power systems require an asynchronous link. For some of these areas the power systems have different nominal frequencies (e.g, 60 Hz and 50 Hz). The prevailing technology for accomplishing an asynchronous interconnection between power systems is high voltage direct current (HVDC) conversion. HVDC conversion is complicated due e.g., to the need to closely coordinate harmonic filtering, controls, and reactive compensation. Moreover, HVDC has performance limits when the AC power system on either side has low capacity compared to the HVDC power rating. Further, HVDC undesirably requires significant space, due to the large number of high-voltage switches and filter banks.
Prior art rotary converters utilize a two-step conversion, having both a fully-rated generator and a fully-rated motor on the same shaft. Rotary converters have been utilized to convert power from AC to DC or from DC to AC. However, such rotary converters do not convert directly from AC to AC at differing frequencies. Moreover, rotary converters run continuously at one predetermined speed (at hundreds or thousands of RPMs), acting as motors that actually run themselves.
Rauhut has disclosed a rotary transformer for coupling multi-phase systems having a small frequency difference. See, for example, U.S. Pat. No. 3,471,708 to Rauhut wherein a non-synchronous rotary machine has stator windings connected to a first three-phase power system grid and rotor windings connected to a second three-power system grid. If the frequency of one system is different from that of the second system, a torque is exerted on the rotor in one direction or the other so as to cause rotation of the rotor at a rotational rate equal to the different between the network frequencies.
A closed loop angular positioning control system which operates a rotary transformer for transferring power from a first electrical system to a second electrical system is disclosed in U.S. Pat. application Ser. No. 08/825,502 filed Mar. 31, 1997 by Runkle et al. entitled "INTERCONNECTION SYSTEM FOR TRANSFERRING POWER BETWEEN ELECTRICAL SYSTEMS", which is incorporated herein by reference. Further, a power flow controller including two rotary transformers connected together and controlled to provide an effective phase shift and voltage magnitude ratio between two electrical areas is disclosed in U.S. Pat. application Ser. No. 08/814,374 filed Mar. 11, 1997 by Larsen et al. entitled "POWER FLOW CONTROL WITH ROTARY TRANSFORMERS", also incorporated herein by reference.